Heusler compounds with non-magnetic spacer layer for formation of synthetic anti-ferromagnets (saf)

ABSTRACT

A device including a multi-layered structure that includes: a first layer that includes a first magnetic Heusler compound; a second layer that is non-magnetic at room temperature and includes both Ru and at least one other element E, wherein the composition of the second layer is represented by Ru1−xEx, with x being in the range from 0.45 to 0.55; and a third layer including a second magnetic Heusler compound. The multi-layered structure may overlay a substrate. The device may include a tunnel barrier overlying the multi-layered structure.

This application is a continuation application claiming priority to Ser.No. 16/271,721, filed Feb. 8, 2019.

TECHNICAL FIELD

The invention is in the field of magnetic random access memory (MRAM),and more particularly, MRAM devices that rely on spin transfer torque,racetrack memory, and hard disk storage.

BACKGROUND

The Heusler compounds are a class of materials having the representativeformula X₂YZ, where X and Y are transition metals or lanthanides, and Zis from a main group element. Due to the chemical distinction between X(or Y) and Z, they form a unique structure defined by the space groupsymmetry L2₁ (or D0₂₂ when they are tetragonally distorted), where fourface-centered cubic structures penetrate each other. The properties ofHeusler compounds are strongly dependent on the ordering of the elementsconstituting the compounds. Thus, the fabrication of high qualityHeusler films typically requires high temperature thermal processes, forexample, deposition at temperatures significantly above room temperatureand/or thermal annealing at high temperatures (400° C. or higher).

SUMMARY

Disclosed herein are highly textured (epitaxial), very smooth, highquality ultrathin bilayer films of Heusler compounds separated by anon-magnetic templating spacer layer, which can be fabricated without athermal annealing process. The templating spacer layer is preferablyformed from a binary alloy of Ru—Al with the B2 structure, the cubicversion of L1₀. The templating layer can be deposited at roomtemperature and is ordered (i.e., the formation of alternating atomiclayers of Ru and Al), even in the as-deposited state.

Of particular interest are ultrathin bilayer films of Heusler compoundsdeposited with RuAl templating spacer layers, with these structuresbeing highly epitaxial and ordered. The Heusler compounds disclosedherein form high quality films with excellent magnetic properties,having large perpendicular magnetic anisotropy and square magnetichysteresis loops (with the remanent moment in zero magnetic field beingclose to the saturation moment of each individual layer). Theseadvantageous properties are attributed to the similarity between the B2symmetry of the templating layer and the L2₁ (or D0₂₂) symmetry of theHeusler layers. An important property of the structures made fromultrathin bilayer films of Heusler compounds with an RuAl templatingspacer layer is that the relative orientation of the magnetic moments ofthe two Heusler layers depends on the thickness of the RuAl spacerlayer. For thicknesses in the ranges 4 to 10 Å and 15 to 16 Å. thisrelative orientation of magnetic moments is anti-parallel. Based on thepreviously observed periodicity of oscillatory coupling with elementalCr and Ru layers, a second thickness range of anti-parallel coupling ofmagnetic moments is expected from 15 Å to ˜20 Å. This antiferromagneticoscillatory coupling effect is not observed with a CoAl templatingspacer layer, where the magnetic moments of the two Heusler-containinglayers are always parallel to each other, independent of the spacerlayer thickness, up to at least 15 Å and even to 20 Å or more.

The most important characteristic of the templating spacer layer is thatit is composed of elements that are found in Heusler compounds. Thus,for example, any intermixing or diffusion of the Al from a RuAlunderlayer into the Heusler-containing layer does not significantlychange the properties of the Heusler-containing layer, since Al is fromthe class of “Z elements” (see Background) from which the Heuslercompounds are formed. Similarly, a layer that partially replaces Al withother Z elements, such as Ga, Ge and/or Sn, would be suitable as atemplating spacer layer.

Another important property of the templating spacer layer is that it canreplicate the induced physical ordering of the first Heusler-containinglayer and in turn promote the ordering of the second Heusler-containinglayer, which is grown on top of the templating spacer layer. The firstHeusler-containing layer will inevitably have terraces (see FIG. 1),with atomic steps between neighboring terraces that separate a terracewith a surface formed from X (or Y) from a terrace formed from XZ (seeBackground for a discussion of X, Y, and Z). Due to the chemicalaffinity of X (or Y) to Al, and of Z to Ru, the first Heusler-containinglayer will promote the ordering of the templating spacer layer and, forthe same reasons mentioned previously, will in turn promote ordering inthe second Heusler-containing layer at modest temperatures (even at roomtemperature), as illustrated in FIG. 1.

One embodiment of the invention is a device that includes amulti-layered structure. This structure includes a first magneticHeusler compound, a second layer that is non-magnetic at roomtemperature and which is in contact with and overlies the first layer,and a third layer that is in contact with and overlies the second layer.The third layer includes a second magnetic Heusler compound. The secondlayer comprises both Ru and at least one other element E, in which thecomposition of the second layer is represented by Ru_(1-x)E_(x), with xbeing in the range from 0.45 to 0.55. More preferably x is in the rangefrom 0.47 to 0.53. In a preferred embodiment E is aluminum, and thefirst and third layers each have a thickness of less than 5 nm or even 3nm. The first and second Heusler compounds may be advantageouslyindependently selected from the group consisting of Mn_(3.1-x)Ge,Mn_(3.1-x)Sn, and Mn_(3.1-x)Sb, with x being in the range from 0 to 1.1in the case of Mn_(3.1-x)Sb, and with x being in the range from 0 to 0.6for Mn_(3.1-x)Ge and Mn_(3.1-x)Sn. The first and/or the second Heuslercompounds may be a ternary alloy, e.g., of the formMn_(3.1-x)Co_(1.1-y)Sn, wherein x≤1.2 and y≤1.0. In some embodiments,the magnetic moments of the first and third layers are substantiallyparallel (or alternatively perpendicular) to the interfaces between (i)the second layer and (ii) the first and third layers, respectively.Also, the magnetic moments of the first and third layers may besubstantially anti-parallel to each other, in which the second layer hasa thickness in the range of 6 to 10 Å. Preferred embodiments of thedevice may be used as a memory element or as an element of a racetrackmemory device.

Another embodiment of the invention is a device that includes asubstrate, a multi-layered structure overlying the substrate, in whichthe structure includes a first layer, a second layer, and a third layer.The first layer includes a first magnetic Heusler compound, the secondlayer is non-magnetic at room temperature and comprises Ru and E (inwhich E comprises at least one other element that includes Al, thecomposition of the second layer being given by Ru_(1-x)E_(x), with x inthe range from 0.45 to 0.55), and the third layer includes a. secondmagnetic Heusler compound. The device further includes a tunnel barrieroverlying the multi-layered structure and an additional magnetic layerin contact with the tunnel barrier, in which the additional magneticlayer has a switchable magnetic moment. In a preferred embodiment, atleast one of the first and third layers includes Co.

Yet another embodiment of the invention is a device that includes amulti-layered structure comprising a first layer, a second layer, and athird layer, in which the first layer is magnetic and includes a Heuslerand/or a L1₀ compound, the second layer is non-magnetic at roomtemperature and comprises Ru and E (with E including at least one otherelement that includes Al, the composition of the second layer beingrepresented by Ru_(1-x)E_(x), with x in the range from 0.45 to 0.55),and the third layer is magnetic and includes a Heusler and/or a L1₀compound. The L1₀ to compound may be advantageously selected from thegroup consisting of MnGa, MnAl, FeAl, MnGe, MnSb, and MnSn alloys.Preferred embodiments of the device may be used as memory element or asan element of a racetrack memory device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. Illustration of the templating concept using a bilayer ofHeusler compounds separated by a RuAl spacer layer, X, X′ representtransition metal elements, and Z, Z′ represent main group elements.

FIG. 2A. XRD scans for three different stacks on a MgO/MgO(001)substrate. The templating layers were deposited at room temperature andannealed in situ at 400° C. in vacuum.

FIG. 2B. P-MOKE hysteresis loops at room temperature for the stacks ofFIG. 2A.

FIG. 3A. P-MOKE hysteresis loops from samples having a templating layerof 50 Å CoAl/10 Å RuAl in contact with a 20 Å thick Mn_(2.3)Sb Heuslerlayer.

FIG. 3B. P-MOKE hysteresis loops from samples having a templating layerof 50 Å CoAl/10 Å RuAl in contact with a 12 Å Mn₃Ge Heusler layer.

FIG. 4. P-MOKE hysteresis loop from the illustrated Heusler stack, whichincludes an 8 Å RuAl spacer layer. The magnetic moments of the twoHeusler-containing layers at different positions in the hysteresis loopare indicated with arrows.

FIG. 5. P-MOKE hysteresis loops of Heusler stacks (see left hand side ofFIG. 5) having respective RuAl spacer layers of thicknesses 0, 2, 6, 8,10, and 16 Å.

FIG. 6A. P-MOKE hysteresis loops of Heusler stacks (see stack structureshown in FIG. 5) interposed with spacer layers of RuAl.

FIG. 6B. P-MOKE hysteresis loops of Heusler stacks (see stack structureshown in FIG. 5) interposed with spacer layers of CoAl.

FIG. 7A. P-MOKE hysteresis loops of Heusler stacks (see stack structureshown in FIG. 5) including respective RuAl spacer layers whosethicknesses are 0, 8, and 14 Å. The V_(r), V_(HighField), andH_(SpinFlop) are illustrated.

FIG. 7B. Variation of H_(SpinFlop) as a function of the thickness of theRuAl spacer layer.

FIG. 7C. Remanent Kerr signal voltage divided by Kerr signal voltage atsaturation, as a function of the thickness of the RuAl spacer layer.

FIG. 8. Anomalous Hall Effect (AHE) measurement for Heusler stacks (seestack structure shown in FIG. 5) having a RuAl spacer layer.

FIG. 9. Normalized Anomalous Hall Effect resistivity ratio determinedfor the remanent (ρ_(r) ^(AH)) and saturated (ρ_(Sat) ^(AH)) state forHeusler stacks (see stack structure shown in FIG. 5) of varying RuAlspacer layer thickness.

FIG. 10A. AHE signal from various Heusler stacks (see stack structureshown in FIG. 5, but with CoAl replacing the RuAl as the spacer) thatinclude a CoAl spacer layer.

FIG. 10B. Comparison of normalized AHE resistivity ratio (ρ_(r)^(AH)/ρ_(Sat) ^(AH)) for Heusler stacks including RuAl and CoAl spacerlayers.

FIG. 10C. Comparison of normalized AHE resistivity ratio (ρ_(r)^(AH)/ρ_(Sat) ^(AH)) and normalized (V_(r)/V_(sat)) from P-MOKE for theHeusler stacks including RuAl.

FIG. 11. Illustration of a magnetic tunnel junction device thatincorporates the synthetic anti-ferromagnet structure having the Heuslerlayers (and the corresponding spacer layer) described herein.

DETAILED DESCRIPTION

New magnetic materials are needed to allow for scaling of STT-MRAM (spintransfer torque-magnetic random access memories) beyond the 20 nm node.These materials must have very large perpendicular magnetic anisotropy(PMA) and, for integration purposes, be compatible with conventionalCMOS technologies. Such magnetic materials form electrodes of magnetictunnel junction (MTJ) based memory elements. An important mechanism forswitching the state of the MTJ element is passing spin polarizedtunneling current through the MTJ. The magnitude of this current islimited by the size of the transistors used to provide the writecurrent. This means that the thickness of the electrode must besufficiently small that it can be switched by the available current. Formagnetization values of ˜1000 emu/cm³, the electrode must have athickness that does not exceed approximately 1 nm.

Recently it has been shown that using templating layers, such as CoAl,CoGa, CoSn, or CoGe, it is possible to deposit an ultrathin Heuslerlayer (thickness of ˜1 nm) having bulk like magnetic properties (seeU.S. patent application Ser. No. 15/660,681 filed 26 Jul. 2017 and U.S.Pat. No. 10,177,305 issued 8 Jan. 2019). These ultrathin Heuslercompound films of even a single unit cell thickness showed perpendicularmagnetic anisotropy and square magnetic hysteresis loops, making themcandidate materials for use in STT-MRAM and racetrack memoryapplications. In both technology applications, a syntheticanti-ferromagnet (SAF) is advantageously used. In an STT-MRAMapplication, the reference layer includes an SAF structure, since thisstructure has very small fringing fields, which are the primary cause ofthe offset fields observed in the measured hysteresis loops of thestorage layer. In a race-track memory, the domain wall velocities innano-wires of an SAF structure are significantly higher than those inthe nano-wires of conventional ferromagnets. The SAF structures fromconventional ferromagnets use Ru as the non-magnetic spacer layer. Thefamily of tetragonal Heusler compounds, which include Mn₃Z with Z=Ge,Sn, and Sb, have a layered structure of alternating layers of Mn—Mn andMn—Z. The use of a known elemental spacer layer (e.g., using Ru alone)does not work for structures including two Heusler layers, sinceelemental Ru is unable to replicate the ordering of the Heusler layerunderneath it; thus, it is unable to promote the ordering in the secondHeusler layer grown over the Ru spacer layer.

Disclosed herein is a spacer layer that promotes the formation of an SAFstructure between Heusler layers. It is shown that a RuAl alloy spacerlayer having the CsCl structure induces anti-ferromagnetic couplingbetween two tetragonal Heusler compound layers separated by that spacerlayer.

RuAl Templating Layer

Single crystal epitaxial films of Ru_(1-x)Al_(x) alloy were grown bydc-magnetron sputtering in an ultra-high vacuum (UHV) chamber with abase pressure of ˜2×10⁻⁹ Torr. Argon was used as the sputter gas at atypical gas pressure of 3 mTorr. The MgO buffer layer was prepared bydepositing 20 Å thick MgO at room temperature using ion-beam deposition(IBD) from a MgO target obtained from Kojundo Chemical Laboratory.Alternately, this MgO buffer layer can be deposited at room temperatureby RF magnetron sputtering from an MgO target (Kojundo ChemicalLaboratory). The templating layers (TL) of CoAl, CoAl/RuAl, or RuAl werethen deposited at room temperature. The TL was annealed at 400° C. for30 minutes in ultra-high vacuum and then cooled to room temperatureprior to deposition of the subsequent layers, which included a Heuslerlayer (20 Å Mn₃Sn) and a cap bilayer of 20 Å MgO and 20 Å Ta. The capbilayer protects the layers underneath it during the exposure of theentire stack to ambient environment. FIG. 2A shows the compositions ofthe CoAl and RuAl layers, which were determined by Rutherfordbackscattering measurements to be Co₅₁Al₄₉ and Ru₅₀Al₅₀, respectively.Although the RuAl layer composition of 1:1 in the current example wasideal, Ru_(1-x)Al_(x), layers with x in the range from 0.45 to 0.55, andmore preferably in the range from 0.47 to 0.53, will show similartemplating effects (thereby, facilitating growth of alternating layers).Even larger deviations would be expected to make growth of thesealternating layer structures difficult or impossible.

X-ray diffraction (XRD) θ-2θ scans in the out-of-plane geometry wereperformed on the films. FIG. 2A shows sets of XRD scans for three films:MgO(001)/20 Å MgO/50 Å CoAl/10 Å RuAl/20 Å Mn₃Sn/20 Å MgO/20 Å Ta;MgO(001)/20 Å MgO/300 Å RuAl/20 Å Mn₃Sn/20 Å MgO/20 Å Ta; andMgO(001)/20 Å MgO/50 Å CoAl/20 Å Mn₃Sn/20 Å MgO/20 Å Ta. The data showthe main CoAl (002) peak at 2θ=˜65.5°, as well as the CoAl (001) peak at2θ=˜31.4°, the RuAl (002) peak at 2θ=˜60.75°, and the RuAl (001) peak at2θ=˜29.3°. The existence of the CoAl (001) and RuAl (001) superlatticepeaks clearly prove that alternating layering of Co and Al is takingplace; likewise, alternating layers of Ru and Al are formed. Even thoughthe templating layers were annealed 400° C. for 30 minutes, it was found(not shown) that this annealing step is not needed to promote the growthof alternating layers of Co/Al and Ru/Al. The x-ray diffraction peakassociated with the substrate was observed for all samples and islabeled as MgO(002). The lattice parameter of the CoAl film is ˜2.86 Å,which is close to that of bulk CoAl in the B2 structure. The latticeparameter of the RuAl film is ˜2.95 Å, which is close to that of hulkRuAl in the B2 structure.

FIG. 2B shows the perpendicular magneto-optic Kerr effect (P-MOKE)signal obtained from these three films as a function of the appliedmagnetic field. The square hysteresis loops for 20 Å Mn₃Sn were obtainedfrom films having, as their templating layers, either the bilayerCoAl/RuAl or the single layer CoAl. The 300 Å RuAl layer showed thealternating layer structure (see XRD data above), yet the 20 Å Mn₃Sn didnot show any magnetic hysteresis loop. This is due to the larger latticemismatch (˜7%) between the RuAl and Mn₃Sn Heusler compounds, whichstrained the Mn₃Sn layer sufficiently that it no longer exhibited PMA.However, these data do indicate that the RuAl templating layer isnon-magnetic. Deposition of a RuAl layer on a CoAl layer strained theRuAl sufficiently that it promoted growth of an ordered, magnetic, 20 ÅMn₃Sn layer. These films were very smooth with root mean square surfaceroughness (r_(rms)) of <2 Å, as measured by atomic force microscopy(AFM).

FIG. 3A shows a P-MOKE hysteresis loop for a Mn_(2.3)Sb Heusler compoundgrown on a RuAl templating layer. The material stack for this sample wasMgO(001)/20 Å MgO/50 Å CoAl/10 Å RuAl/20 Å Mn_(2.3)Sb/20 Å MgO/20 Å Ta.Similarly, FIG. 3B shows a P-MOKE hysteresis loop for a Mn₃Ge Heuslercompound grown on a RuAl templating layer. The material stack for thissample was MgO(001)/20 Å MgO/50 Å CoAl/10 Å RuAl/12 Å Mn₃Ge/20 Å MgO/20Å Ta. The spin polarizations of Mn_(2.3)Sb and Mn₃Ge are opposite ofeach other, as are their Kerr contrasts, which leads to the oppositesign of the P-MOKE signal for these two Heusler layers. This hasinteresting consequences: The P-MOKE signal from a bilayer of Mn_(2.3)Sband Mn₃Ge, while sweeping the field, will be reduced when the twoHeusler layers align independently with the external field and thenbecome parallel. These results show that the RuAl templating layer canpromote ordering within Mn₃Z Heusler compounds (where Z=Ge, Sn, and Sb),such that they have perpendicular magnetic anisotropy.

FIG. 4 includes the P-MOKE hysteresis loop obtained from a sample withtwo different layers of Heusler compounds separated by a non-magneticspacer layer of RuAl. The stack of this sample was MgO(001)/20 Å MgO/50Å CoAl/12 Å Mn₃Ge/t=8 Å RuAl/20 Å Mn_(2.3)Sb/20 Å MgO/20 Å Ta (where “t”represents thickness). Three distinct hysteresis loops are observed, andthe sets of pairs of arrows overlayed in FIG. 4 indicate theorientations of the magnetization of the Mn₃Ge and Mn_(2.3)Sb layers. Athigh applied fields >5 kOe, the magnetizations of the two Heuslercompounds are parallel to each other. At zero applied field, in theremanent state, the magnetizations of the two Heusler compounds areanti-parallel to each other. Thus, the presence of the 8 Å RuAl spacerlayer separating the two Heusler compounds promotes the formation of asynthetic anti-ferromagnet (SAF). This is the first demonstration of anSAF structure based on Heusler compounds. Moreover, the Heuslercompounds, along with their corresponding spacer layer, were depositedat room temperature, with the resulting SAF structure needing nosubsequent annealing. Furthermore, in the case of two layers of Heuslercompounds having magnetic moments that are in-plane, the presence of an8 Å RuAl spacer layer separating these two Heusler layers will also leadto the formation of a synthetic anti-ferromagnet (SAF). More explicitly,here the magnetic moments of the Heusler layers are substantiallyparallel to the interfaces between the Heusler layer and the RuAl spacerlayer separating them. The magnetic moments of the two Heusler layersmay be substantially anti-parallel to each other when the RuAl spacerlayer has a thickness in the range of 6 to 10 Å.

Although the thickness of the Heusler layers within the SAF structureused here was 1-2 nm, it is possible to form SAF structures withsignificantly thicker Heusler layers. For technologically relevant SAFstructures, the thickness of the Heusler layers is expected to be lessthan 5 nm or even less than 3 nm.

FIG. 5 summarizes the P-MOKE hysteresis loops measured from sampleshaving two layers of different Heusler compounds separated by anon-magnetic spacer layer of RuAl (of varying thickness, t). The stackof these samples was MgO(001)/20 Å MgO/50 Å CoAl/12 Å Mn₃Ge/t=0, 2, 6,8, 10, and 16 Å RuAl/20 Å Mn₃Sb/20 Å MgO/20 Å Ta. The samples with t=0and 2 Å RuAl display a single square hysteresis loop indicative offerromagnetic coupling between the Heusler layers. For samples with t=6,8, 10, and 16 Å of RuAl, the coupling between the Heusler layers isanti-ferromagnetic.

FIG. 6A and FIG. 6B compare the P-MOKE hysteresis loops measured onsamples having two layers of Heusler compounds separated by anon-magnetic spacer layer of RuAl (see FIG. 6A) and CoAl (see FIG. 6B).The spacer layer thickness t is varied from 0 to 16 Å. The stack ofthese samples was MgO(001)/20 Å MgO/50 Å CoAl/12 Å Mn₃Ge/t RuAl orCoAl/20 Å Mn₃Sb/20 Å MgO/20 Å Ta. The thickness of the RuAl spacer layervaried from t=0 to 16 Å in 1 Å increments (only some of the data areshown) and the thickness of the CoAl spacer layer was t=0, 4, 6, 7, 8,9, 10, 12, 14 and 16 Å (again, only some of the data are shown forclarity). The hysteresis loops obtained for samples a CoAl spacer layershow a single square hysteresis loop for all CoAl thicknesses studiedhere, which is different than what was obtained for samples having aRuAl spacer layer (discussed in greater detail with respect to FIG. 9and FIG. 10B below). The two Heusler layers separated by a CoAl spacerlayer are coupled ferromagnetically for all thicknesses, and there is noevidence of the formation of an SAF structure.

The dependence of the hysteresis loops for samples with a RuAl spacerlayer are detailed in FIG. 7A, FIG. 7B, and FIG. 7C. FIG. 7A showshysteresis loops from samples having a RuAl spacer layer whose thicknesst was 0, 8, and 14 Å. The P-MOKE signal measured at H=0 kOe is V_(r)(representing the remanent state), and the P-MOKE signal measured at ahigh applied field of ˜1.4 T (not within the displayed region) isV_(HighField) (representing the saturated state), whose magnitude isillustrated with the dark rectangles at a much lower field strength.When the field H is swept, the relative orientation of the magnetizationof the two Heusler layers switches from parallel to anti-parallel. TheH_(SF) depends on the RuAl spacer thickness and is indicated by soliddark circles in FIG. 7A. FIG. 7B shows the variation of H_(SF) with RuAlspacer layer thickness. The sign of H_(SF) indicates the type ofcoupling between the two Heusler layers, which is negative foranti-ferromagnetic coupling and positive for ferromagnetic coupling.Thus, based on the P-MOKE results, the coupling between the two Heuslerlayers is anti-ferromagnetic for t between about 6 and 10 Å as well asfor t between about 16 and approximately 20 Å (see also FIG. 7C); thecoupling is ferromagnetic for the other thicknesses studied. FIG. 7Cshows the ratio of V_(r) to V_(sat) where V_(sat)=V_(HighField) forferromagnetic coupling and V_(sat)=V_(r)+V_(HighField) foranti-ferromagnetic coupling.

The anomalous Hall Effect (AHE) signals for the samples described abovewere measured in a Quantum Design DynaCool apparatus at roomtemperature. These measurements are summarized in FIG. 8 for Heuslerlayers separated by RuAl layers of various thicknesses. The sampleresistivity was measured using a standard Hall measurement geometry inwhich wire was bonded to the four corners of the square sample (10 mm×10mm). The measured resistivity depends on the Lorentz force (which has alinear dependence on applied field (H)) and the AHE. After subtractingbackground from the resistivity data, the AHE signal was determined atzero field, ρ_(r) ^(AH) (remanent, H=0) and maximum applied field,ρ_(Sat) ^(AH) (saturated, H_(sat)). The AHE signal (ρ_(AH)) is oppositefor the two Heusler compounds, consistent with the P-MOKE measurements.The following three equations were used to determine the AHE signal ofthe Heusler compounds in both the remanent (ρ_(AH) ^(H=0)) and saturated(ρ_(AH) ^(Hsat)) state.

ρ_(r) ^(AH)=ρ_(AH) ^(H=0)

ρ_(Sat) ^(AH)=ρ_(AH) ^(Hsat) for a ferromagnet (FM)

ρ_(Sat) ^(AH)=(ρ_(AH) ^(H=0)+ρ_(AH) ^(Hsat)) for an anti-ferromagnet(AFM)

FIG. 9 shows the variation of the ratio (ρ_(r) ^(AH)/ρ_(Sat) ^(AH)),determined from the AHE as a function of the RuAl spacer layerthickness. These results indicate that at low spacer layer thickness(t_(RuAl)<4 Å), the coupling between the Heusler layers isferromagnetic. For a spacer layer thickness in the range of 4 to 11 Å,the coupling between the Heusler layers is anti-ferromagnetic. Thecoupling is ferromagnetic again for spacer layer thicknesses in therange 12 to 15 Å.

FIG. 10A shows the AHE for Heusler layers having CoAl as the spacerlayer. The ratio (ρ_(r) ^(AH)/ρ_(Sat) ^(AH)) for these samples is ˜1 andis independent of the thickness of the CoAl layer, as shown in FIG. 10B,which compares the ratio (ρ_(r) ^(AH)/ρ_(Sat) ^(AH)) for two spacerlayers, CoAl and RuAl. The coupling between the two Heusler compounds isferromagnetic when CoAl is used as the spacer layer. The ratios (ρ_(r)^(AH)/ρ_(Sat) ^(AH)) or (V_(r)/V_(sat)), as determined from AHE andP-MOKE, are compared in FIG. 10C and found to be largely consistent.Thus, a RuAl spacer layer enables anti-ferromagnetic coupling betweentwo Heusler layers, resulting in a synthetic anti-ferromagnet for anappropriate RuAl thickness.

The structural ordering of ultrathin layers is likely due to thedistinct chemical properties of the elements Ru and Al in the templatingspacer layer. As an alternative to Al, Al alloys such as AlSn, AlGe,AlGa, AlGaGe, AlGaSn, AlGeSn, and AlGaGeSn may be employed. Binary (X=Y)and ternary Heusler alloys consist of two/three different types ofatoms, respectively. In X₂YZ Heuslers, the Z main group elementtypically has high chemical affinity for X and Y. In this context, theformation of an ordered structure should take place, irrespective of thechoice of Z. An example of a ternary Heusler compound that could be usedis Mn_(3.1-x)Co_(1.1-y)Sn, wherein x≤1.2 and y≤1.0. The Heusler SAFstructure could comprise a ternary Heusler compound as either the firstHeusler layer, the second Heusler layer, or both Heusler layers.

Mn_(2.3)Sb is also considered to be part of the family of L1₀ compoundsand hence the results discussed above indicate that the RuAl templatingspacer layer would also be effective in inducing SAF ordering betweentwo L1₀ compounds (whose constituent elements include one transitionmetal element and a main group element). Other candidate L1₀ compoundsare MnAl alloys, MnGa alloys, MnSn alloys, MnGe alloys, and FeAl alloys.

The structures described herein lend themselves to a variety ofapplications, including MRAM elements and a racetrack memory device,such as that described in U.S. Pat. No. 6,834,005, issued Dec. 21, 2004and titled “Shiftable magnetic shift register and method of using thesame,” which is hereby incorporated herein. One such MRAM element isshown in FIG. 11. As with MRAM elements generally, a tunnel barrier issituated between two magnetic electrodes, one of which has a fixedmagnetic moment and the other of which has a magnetic moment that isswitchable, thereby permitting the recording and erasing of data. UnlikeMRAM elements of the prior art, however, the magnetic layer of FIG. 11having the fixed magnetic moment (pinning layer) comprises Heuslerlayers separated by a non-magnetic spacer (such as those describedherein). An optional (second) pinning layer may be advantageouslyemployed for even better performance.

The templating layer of FIG. 11 is, as described previously herein, amulti-layered structure that is non-magnetic at room temperature, andwhich comprises alternating layers of Co and at least one other elementE (preferably Al or Ga; or Al alloyed with Ga, Ge, Sn or any combinationthereof, such as AlSn, AlGe, AlGa, AlGaGe, AlGaSn, AlGeSn, andAlGaGeSn). The composition of this structure is represented byCo_(1-x)E_(x), with x preferably being in the range from 0.45 to 0.55(close to a ratio of 1:1, to facilitate growth of alternating layers).At high Co concentrations, the Co-E alloy would be magnetic; also,deviations from 1:1 would also make growth of these structures difficultor impossible. An optional seed layer may be interposed between thesubstrate and the templating layer. Overlying the templating layer is aHeusler SAF structure which includes a first Heusler compound, e.g.,Mn₃Ge, Mn₃Sn, or Mn₃Sb. Although these three binary Heusler compoundsare mentioned with respect to their ideal stoichiometry representation(i.e., Mn₃Z where Z=Ge or Sn or Sb), their stoichiometry can vary over alimited range as discussed below. For Mn_(3.1-x)Ge and Mn_(3.1-x)Sn, xcan be in the range from 0 to 0.6; for Mn_(3.1-x)Sb, x can be in therange from 0 to 1.1. A spacer layer that includes both Ru and at leastone other element E (having a composition represented by Ru_(1-x)E_(x),with x being in the range from 0.45 to 0.55) is in contact with thefirst Heusler layer. A second Heusler layer chosen independently fromthe group Mn₃Ge, Mn₃Sn, and Mn₃Sb is in contact with the spacer layer.The magnetic moments of the first and second Heusler layers are fixedand anti-parallel to each other. An optional, second pinning layer maybe used to increase performance, and may include Fe, a CoFe alloy, orCo₂MnSi.

The tunnel barrier is preferably MgO (001), although other(001)-oriented tunnel barriers may be used, such as CaO and LiF.Alternatively, an insulator with a spinel structure such as MgAl₂O₄ canbe used as a tunnel barrier; its lattice spacing could be tuned bycontrolling the Mg—Al ratio, which would result in better latticematching with the Heusler compounds (more preferably, the Mg—Alcomposition is Mg_(1-z)Al_(2-z)O₄, wherein −0.5<z<0.5). The switchablemagnetic electrode overlying the tunnel barrier may comprise Fe, a CoFealloy, or a CoFeB alloy, for example. The capping layer may comprise Mo,W, Ta, Ru, or a combination thereof. Current may be induced by applyinga voltage between the two magnetic electrodes, which are separated bythe tunnel barrier.

Certain structures described herein may also be used in racetrack memorydevices. In this case, the racetrack may be a nanowire that includes asubstrate, an optional seed layer, a templating layer, and a firstmagnetic layer that includes two Heusler compounds separated bynon-magnetic spacer layer. (See the discussion above with respect toFIG. 11 for possible compositions of these layers. Note that in aracetrack memory device, the tunnel barrier and the switchable magneticlayer shown in FIG. 11 would not normally be present; however, in thiscase the first magnetic layer shown in FIG. 11 would have a magneticmoment that is switchable rather than fixed.) Magnetic domain walls maybe moved along this racetrack, as described in U.S. Pat. No. 6,834,005.Data may be read out of (and stored in) the racetrack by interrogating(or changing) the orientation of the magnetic moment of the magneticmaterial between adjacent domain walls within the racetrack.

The various layers described herein may be deposited through any one ormore of several methods, including magnetron sputtering,electrodeposition, ion beam sputtering, atomic layer deposition,chemical vapor deposition, and thermal evaporation.

The invention may be embodied in other specific forms without departingfrom its spirit or essential characteristics. The described embodimentsare to be considered in all respects only as illustrative and notrestrictive. The scope of the invention is therefore indicated by theappended claims rather than the foregoing description. All changeswithin the meaning and range of equivalency of the claims are to beembraced within that scope.

What is claimed is:
 1. A device, said device comprising a multi-layeredstructure, said multi-layered structure comprising: a first layer thatincludes a first magnetic Heusler compound; a second layer that isnon-magnetic at room temperature, said second layer comprising both Ruand at least one other element E, wherein the composition of the secondlayer is represented by Ru_(1-x)E_(x), with x being in the range from0.45 to 0.55; and a third layer including a second magnetic Heuslercompound.
 2. The device of claim 1, wherein the third layer overlays thesecond layer, and wherein the second layer overlays the first layer. 3.The device of claim 1, wherein the third layer is in contact with thesecond layer, and wherein the second layer is in contact with the firstlayer.
 4. The device of claim 1, wherein x is in the range from 0.47 to0.53.
 5. The device of claim 1, wherein the magnetic moments of thefirst and third layers are substantially perpendicular to interfacesbetween (i) the second layer and (ii) the first and third layers,respectively.
 6. The device of claim 1, wherein the first and thirdlayers each have a thickness of less than 5 nm.
 7. The device of claim1, wherein the first and third layers each have a thickness of less than3 nm.
 8. The device of claim 1, wherein the second layer has a thicknessin the range of 6 to 10 Å.
 9. The device of claim 8, wherein themagnetic moments of the first and third layers are substantiallyanti-parallel to each other.
 10. The device of claim 1, wherein thefirst and second magnetic Heusler compounds are independently selectedfrom the group consisting of Mn_(3.1-x)Ge, Mn_(3.1-x)Sn, andMn_(3.1-x)Sb, with x being in the range from 0 to 1.1 in the case ofMn_(3.1-x)Sb, and with x being in the range from 0 to 0.6 forMn_(3.1-x)Ge and Mn_(3.1-x)Sn.
 11. The device of claim 1, wherein thefirst and/or the second magnetic Heusler compound is a ternary Heuslercompound.
 12. The device of claim 11, wherein the ternary Heuslercompound is Mn_(3.1-x)Co_(1.1-y)Sn, wherein x≤1.2 and y≤1.0.
 13. Thedevice of claim 1, wherein the magnetic moments of the first and thirdlayers are substantially parallel to interfaces between (i) the secondlayer and (ii) the first and third layers, respectively.
 14. The deviceof claim 13, wherein the magnetic moments of the first and third layersare substantially anti-parallel to each other, and wherein the secondlayer has a thickness in the range of 6 to 10 Å.
 15. The device of claim1, wherein E is Al, an AlGe alloy, or an AlGa alloy
 16. The device ofclaim 1, wherein E includes an alloy selected from the group consistingof AlSn, AlGe, AlGaGe, AlGaSn, AlGeSn, and AlGaGeSn.
 17. The device ofclaim 1, comprising a substrate underlying the multi-layered structure.18. The device of claim 17, said device further comprising a tunnelbarrier overlying the multi-layered structure, thereby permittingcurrent to pass through both the tunnel barrier and the multi-layeredstructure.
 19. The device of claim 18, wherein the tunnel barrier is MgOor Mg_(1-z)Al_(2-z)O₄, wherein −0.5<z<0.5.
 20. The device of claim 18,said device further comprising an additional magnetic layer in contactwith the tunnel barrier.
 21. A method, comprising: using the device ofclaim 20 as a memory element.
 22. A method, comprising: using the deviceof claim 17 as an element of a racetrack memory device.
 23. A device,said device comprising: a multi-layered structure overlying a substrate,said multi-layered structure comprising a first layer, a second layer,and a third layer, wherein: the first layer includes a first magneticHeusler compound, the second layer is non-magnetic at room temperatureand comprises Ru and E, wherein E comprises at least one other elementthat includes Al, the composition of the second layer being given byRu_(1-x)E_(x), with x in the range from 0.45 to 0.55; and the thirdlayer includes a second magnetic Heusler compound; and a tunnel barrieroverlying the multi-layered structure.
 24. The device of claim 23, saiddevice further comprising an additional magnetic layer in contact withthe tunnel barrier, wherein the additional magnetic layer has aswitchable magnetic moment.
 25. The device of claim 24, said devicefurther comprising a capping layer in contact with the additionalmagnetic layer.